The recurring nature of influenza epidemics and pandemics is a consequence of the genetic and antigenic variability of the virus. This changeability in turn arises from mutations introduced by the error prone polymerase and the shuffling of viral gene segments among strains in the process of reassortment. The importance of reassortment among human and animal influenza viruses to the genesis of pandemic strains has been appreciated for many years. More recent phylogenic and epidemiological studies have furthermore established a significant role for reassortment between seasonal influenza viruses in the generation of novel epidemic strains. Nevertheless, little is known about how efficiently and under what circumstances reassortment occurs in a host co-infected with two influenza viruses. Through the proposed research we plan to address this gap in knowledge. Using both cell culture and guinea pig models, we will first determine the frequency with which reassortment occurs in the absence of selective pressures. We will then introduce systematically a number of variables, such as pre-existing immunity or a time interval between infections, and determine whether each increases or decreases the number of reassortant progeny arising from the co-infection. In this way, we will establish the parameters that govern the process of influenza virus reassortment. Since reassortant strains will be epidemiologically significant only if they are transmitted to additional hosts, we will also evaluate the size of the bottleneck created by influenza virus transmission. Thus, through carefully controlled experimental studies, we propose to reveal novel insights in the mechanisms governing influenza virus evolution and the emergence of epidemic and pandemic strains.